Various electromagnetic techniques resist to perform surveys of subterranean structures underneath a surface for identifying structures of interest. Examples of structures of interest in the subterranean structure include subsurface resistive bodies, such as oil-bearing reservoirs, gas injection zones, and fresh-water aquifers. One survey technique is the controlled source electromagnetic (CSEM) survey technique, in which an electromagnetic (EM) transmitter (typically towed by a sea vessel in a marine environment) is used to generate electromagnetic signals. Surveying units (or receivers) containing electric and magnetic field sensors are deployed on the sea floor within an area of interest to make measurements (of EM wavefields) from which a geological survey of the subterranean structure underneath the surface can be derived.
Through the use of the CSEM technique, a high-resolution mapping of changes in resistivity associated with the presence of oil and hydrocarbon is possible. Measurements taken by the EM receivers are interpreted in such a way that a prediction of the presence and location of oil and hydrocarbon in the sedimentary layers of the subterranean structure can be made.
In a shallow water environment, ghosting and water-layer multiple reflections can occur due to the presence and proximity of the air-water interface to the surveying units. Ghosting and water-layer multiple reflections arise because water has a substantially different resistivity from the air above the water surface. Furthermore, water typically has a substantially different resistivity from the earth formations underneath the sea floor. When an EM transmitter located in sea water is activated, the downwardly radiating EM energy from the EM transmitter passes through the sea floor and into the subterranean structure. Some of the EM energy is reflected by resistivity changes of certain layers in the subterranean structure. The reflected EM energy (in the form of reflected EM signals) travels generally upwardly through the subterranean structure, and is ultimately detected by EM receivers at the sea floor. After the reflected signal reach these receivers, the reflected signals continue to travel upwardly through the sea water until they reach the sea surface. The sea surface reflects a large part of the upwardly traveling EM signals. Therefore, a substantial part of the upwardly traveling EM signals will reflect back from the sea surface, and travel downwardly once again.
The sea surface reflected downwardly traveling EM signals would also be shifted in phase from the upwardly traveling EM signals. A surface-reflected, downwardly traveling EM signal is commonly referred to as a “ghost” signal. Ghost signals are also measured by the receivers at the sea floor. In addition, downwardly traveling EM signals, whether reflected from the sea surface or transmitted directly by the EM transmitter source, may also reflect from the sea floor and travel back upwardly. EM signals traveling between the sea surface and the sea floor may reflect from both the sea surface and the sea floor a number of times before such signals are attenuated. The multiple reflections of EM signals by the sea surface and sea floor result in the EM receivers detecting multiple versions of the same signals (resulting in the water-layer multiple reflections phenomenon). These reverberations (that cause ghosting and water-layer multiple reflections) can have substantial amplitudes within the total EM fields measured by the receivers, which can cause masking of field signals that are reflected from subterranean structures. The presence of ghosting and multiple reflections make it difficult to accurately identify subterranean structures and compositions based on collected CSEM survey data.